Detection of flaws such as particles, holes, bumps, pits or fingerprints on a surface having diffractive features, such as on a semiconductor wafer, a photolithographic mask used in modern semi-conductor lithography, or any other defect on a patterned surface hereinafter generically referred to as a "plate", is critical to maintaining a high level of quality control.
A system which accomplishes this function is disclosed in U.S. Pat. No. 4,943,734 assigned to the same assignee as the instant application, which is incorporated herein by reference in its entirety. That system utilizes a laser which provides a beam of visible, blue laser light (at the wavelength of 488 nm) that is scanned across the entire surface of the plate. The angular intensity distribution sensed by an array of detectors in response to the illumination at each point on the plate surface is used to determine the location and size of flaws on the plate surface. This system has several shortcomings, however.
With the decreasing size of elements in semiconductor devices, for example, it has become increasingly more important to detect smaller flaws on plates under inspection. The wavelength of the light beam used to probe the surface under inspection is one of the parameters that limits the size of the smallest flaws which can be detected. This is because the optical scattering cross-section decreases sharply as the wavelength increases (for flaws that are smaller in size compared to the beam wavelength). Thus, the wavelength of the laser beam limits the smallest size particle which the system is capable of detecting. Additionally, the wavelength of the laser beam limits the size of the beam spot that can be projected onto the surface under inspection because as the wavelength of the laser beam increases so does the size of the beam spot that can be focussed onto the surface and therefore the optical power density on the surface decreases. A larger beam spot results in decreased sensitivity, as a smaller portion of the beam is affected by flaws on the surface.
Because the system of U.S. Pat. No. 4,943,734 uses a scanned beam, it is not possible to reduce the beam size without prohibitively increasing the cost of both the scanning and detection optics. Thus the sensitivity and resolution of this system are limited by the 488 nm wavelength of the laser beam. This is because this system utilizes transmissive optics to focus the beam onto the surface. In order to focus a smaller beam onto the surface, larger lenses must be used which require larger lens holders. Thus, to decrease the beam spot size large, bulky and more expensive optics are required. Such optics are more difficult to scan.